What is the most self-descriptive way to define a function that calculates sales tax on a purchase?

def tax(my_float):
”Calculates the sales tax of a purchase. Takes in a float representing the subtotal as an argument and returns a float representing the sales tax.”
pass
def tx(amt):
”Gets the tax on an amount.”
def sales_tax(amount):
”Calculates the sales tax of a purchase. Takes in a float representing the subtotal as an argument and returns a float representing the sales tax.”
def calculate_sales_tax(subtotal):
pass